1. Field of the Invention
The present invention relates to a dry etching method and a diffractive optical element (DOE), and more particularly to a dry etching method achieving a stable etching rate as well as a DOE manufactured with that method.
2. Description of the Background Art
In recent years, electronic components and devices used in a portable telephone, a personal computer or the like have been reduced in size, and finer and higher-speed drilling has been demanded. A DOE is a key device for meeting such demand. Unlike a conventional optical element utilizing refraction and reflection, the DOE makes use of diffraction phenomenon of light for directly controlling a phase, so that, for example, the DOE can serve as an optical element of which application to extensive fields such as a multi-point beam splitting function and the like can be expected.
For example, the DOE is utilized as a laser optical element. Specifically, the DOE is irradiated with one laser beam for material processing, and the laser beam is split into multiple beams for irradiating multiple points in order to simultaneously drill a plurality of holes, thus realizing high speed in drilling fine holes.
In order to manufacture a DOE, a dry etching method capable of accurate and micro processing is employed. FIG. 4 is a schematic diagram of an exemplary dry etching apparatus used in a conventional dry etching method. It is noted that the same or corresponding elements have the same reference characters allotted in the drawings of the subject application. The dry etching apparatus includes RF power supplies 1a, 1b, an ICP coil 2, a chamber 3, a conductor 6 implemented by a conductive Si wafer, an electrode 7 cooled by He gas or the like, and a blocking capacitor 8. An insulative substrate 4 serving as a material for the DOE is attached to conductor 6 by means of a vacuum grease 5, which is in turn placed in chamber 3 in such a manner that conductor 6 achieves electric, intimate contact with cooled electrode 7.
Then, a gas is supplied to chamber 3, and positive ions in plasma 9 generated as a result of application of high-frequency power by means of RF power supplies 1a, 1b collide with a surface of insulative substrate 4 because of intense electric field in a sheath region 10 created above insulative substrate 4. Dry etching of insulative substrate 4 is thus performed.
In dry etching, a temperature of the insulative substrate is increased due to collision or chemical reaction of positive ions with the insulative substrate. Therefore, if a photoresist is used as a mask for the insulative substrate, the photoresist is baked due to the increase in the temperature of the insulative substrate, which results in failure in removing the photoresist, or damage or roughness of the surface of the insulative substrate. To address this problem, insulative substrate 4 is attached to conductor 6 by means of vacuum grease 5 as shown in FIG. 4, and conductor 6 is brought in electric, intimate contact with cooled electrode 7 by utilizing electrostatic attraction or the like, so that entire insulative substrate 4 is uniformly cooled through conductor 6 and increase in the temperature of insulative substrate 4 is suppressed.
In the conventional dry etching method, however, an etching rate has not been stable. Preferably, a plurality of insulative substrates are simultaneously dry-etched from a viewpoint of efficiency, however, there has been variation in etching rates as well as in etching depths of insulative substrates.